The present invention relates to a vibrator assembly and, in particular, to a vibrator assembly which is capable of producing such low vibrations as 16 Hertz.
The vibrator assembly as herein disclosed has particular use in the apparatus disclosed in U.S. Pat. Nos. 4,181,025 through 4,181,029 and 4,735,089 which are all assigned to the same assignee of this document. When a pneumatic vibrator, such as described in these patents, is attached to a resonating vibrating table, none of the table resonances below the fundamental frequency of the vibrator are excited. Typical standard vibrators start to operate at about 26 Hertz, including a vibrator having a single cylindrical piston with an elastomeric impactor at one end and an air spring at its other end, and with a single air inlet and four outlets. Such a vibrator has been used and sold over a year prior to the filling of this document in the United States Patent and Trademark Office. The frequency spectra generated by such impacting vibrators are a series of acceleration lines starting at the fundamental frequency and occurring at each harmonic up to the roll off point, for example, 30, 60, 90 . . . 2000 Hertz.
It has been found desirable, and sometimes necessary to begin operation at vibrations below 26 Hertz, such as around 16 Hertz for various reasons. Primarily, it is desired to lower the fundamental resonance of a vibration table such as described in the above patents for testing at lower vibrational frequencies. In the area of screening, better test results can be obtained from disturbances at lower frequencies. It has been discovered that some faults in equipment arise only at such low frequencies and are not discernable at higher frequencies. Secondly, by starting at a lower frequency, the harmonics can be more closely spaced. For example, at a harmonic beginning at 16 Hertz, the harmonics would progress at 16, 32, 48 . . . 1000 Hertz, which results in more energy at the lower frequencies and more low frequency table resonances being excited. Thus, the resonances on the table and on the equipment under test are more evenly excited.